I don't want dark and gritty. I also don't want a series about War. Too many sci-fi series seem to be focused on war and conflict. If I want human misery and pain with lots of casual sex I can watch "The Expanse". If ST: Disc fails it will be because people are tired of pessimism and want some optimism.
So many things wrong with this statement. But I'll try to run them down:
1) We don't know what Discovery is like since we haven't seen it.
2) So we don't know if it's pessimistic
3) and we don't know if the focus is on the war with the Klingons or if that is just the impetus of the story
4) a war is not always portrayed as pessimistic, dark, and gritty. Some war stories can illustrate the triumph of the human spirit, or the value of hope and courage.
5) even if Trek is pessimistic and gritty, it doesn't mean it will fail.
6) I'm not sure where you get the idea that people are "tired of pessimism and want optimism." Some of the best and most popular films and TV shows of the past 10 years are dark, bleak, gritty, and depressing. Here's a few examples from the past 12 months: Game of Thrones, Westworld, Logan, Dunkirk, Better Call Saul, True Detective, Fargo, Twin Peaks... I could go on.
It's not a zero sum game either, we can have fun movies and shows and less optimistic movies and shows at the same time, and they can both be successful.
I too want an optimistic Trek, but saying it will fail if it's pessimistic is simply ludicrous.
All audiences really want compelling stories with solid acting, engaging characters and good drama or comedy. In my opinion, if Discovery doesn't succeed it will be because it fails at one or more of those things.